1. Field of the Invention
The present invention relates to an antenna device and a portable radio communication device, and particularly to an antenna device and a portable radio communication device capable of reducing electromagnetic waves which are generated therefrom and are to be absorbed into a human body.
2. Description of Related Art
Recently, portable data transmitting/receiving devices capable of transmitting/receiving information by radio communication are significantly developed. Of the portable data transmitting/receiving devices, portable radio communication devices for use in the Cellular Telephone System and Personal Communication System etc. are spreading rapidly.
As the portable radio communication devices spread rapidly, the number of communication lines in one radio communication system becomes insufficient. So, a radio communication system which shares another frequency band with another radio communication system is being under consideration to secure necessary communication lines. Thus, as the portable radio communication devices have been significantly reduced in size and weight, portable radio communication devices which can utilize two kinds of radio communication systems are being developed.
Generally, a portable radio communication device has an antenna for transmitting/receiving signals. Actually, whole the conductive portions in the portable radio communication device work as antennas, and the main body of the portable radio communication device other than the antenna portion also generates electromagnetic waves. So, it is required that, of the electromagnetic waves generated from the portable radio communication device, those to be absorbed into a human body should be suppressed. Specifically, of the electromagnetic waves generated from the portable radio communication device in use, amount of electromagnetic waves to be absorbed into a specific portion of a human body (radiation to a human body), particularly a head portion, per unit-time per unit-weight is defined as local average SAR (Specific Absorption Rate), and the maximum value of the local average SAR is required to be not more than a prescribed value.
So as to reduce the maximum value of the local average SAR to be absorbed into a human body, a conductive plate of a predetermined shape may be used. In this case, the conductive plate has its one end connected to a ground conductor which works as an antenna to form a short circuit, and has its other end electrically opened from the ground conductor. As a result, input impedance of the electrically opened end becomes approximately infinite. At this time, high-frequency current flowing to the ground conductor is suppressed, and thus amount of radiation of the electromagnetic waves is reduced.
FIG. 1 shows a schematic view of a portable radio communication device 30, which can reduce the maximum value of the local average SAR. The portable radio communication device 30 includes a circuit board (not shown) necessary for performing radio communication, a shield case 31 as a ground conductor which shields the circuit board, a conductive plate 32, an antenna feeding portion 33, and an antenna 34. The circuit board, shield case 31, and conductive plate 32 are enclosed by a housing (not shown) made of nonconductive material. The conductive plate 32 and shield case 31 are connected by a conductor 35 to form a short circuit.
Since the circuit board is shielded by the shield case 31, various circuits including a transmitting/receiving circuit for communicating with a base station which are mounted on the circuit board do not have bad effects upon. each other, and also do not have bad effects upon the antenna 34 and other devices.
The transmitting/receiving circuit on the circuit board in the shield case 31 generates transmission signals of a predetermined signal form, and sends the transmission signals to the antenna 34 via the antenna feeding portion 33. Then, the antenna 34 transmits the transmission signals to the base station. The antenna 34 receives reception signals from the base station, and sends the reception signals to the transmitting/receiving circuit via the antenna feeding portion 33. Then, the transmitting/receiving circuit performs processing for the reception signals such as demodulating.
The antenna 34 is a rod antenna made of conductive wire materials, or a helical antenna made of conductive wire materials wound spirally. Otherwise, the antenna 34 may be an antenna of various types such as a stretch type antenna combining the rod antenna and helical antenna. When the portable radio communication device 30 performs radio communication, since the high-frequency current flows to the shield case 31 via the antenna feeding portion 33, not only the antenna 34 but also the shield case 31 as a ground conductor for the circuit board works as an antenna. That is, whole the portable radio communication device 30 works as an antenna.
When the portable radio communication device 30 is used, the user comes into contact with a speaker of the portable radio communication device 30. Since the shield case 31 as a ground conductor for the circuit board which is located behind the speaker also works as an antenna and radiates electromagnetic waves, there will be formed a portion where the value of the local average SAR becomes maximum around an ear of the user which comes into contact with the speaker, and this portion will be referred to as a hot spot.
The portable radio communication device 30 has the conductive plate 32 arranged such that the speaker (not shown) faces the conductive plate 32, and the conductive plate 32 and a front surface 31a of the shield case 31 are approximately parallel with each other with a slight interval therebetween. The interval between the conductive plate 32 and the front surface 31a of the shield case 31 depends on a radio communication frequency, and the portable radio communication device 30 can adjust the frequency bandwidth in accordance with the interval.
The conductive plate 32 has its one end along the longitudinal direction connected to the shield case 31 to form a short circuit via the conductor 35, and has its other end electrically opened from the shield case 31. The length L5 between the short circuit forming end and the electrically opened end is set to be a quarter of the wavelength of the radio communication frequency.
Accordingly, the impedance between the conductive plate 32 and the shield case 31 becomes close to zero at the short circuit forming end, while becoming approximately infinite at the electrically opened end. Thus, the high-frequency current has difficulty in flowing from the antenna feeding portion 33 to the conductive plate 32 and the shield case 31.
As has been described, as an example to reduce the maximum value of the local average SAR to be absorbed into a human body, the portable radio communication device 30 mounts a conductive plate 32 thereto, and reduces the amount of radiation of the electromagnetic waves from the conductive plate 32 and shield case 31. Thus, the local average SAR at the hot spot can be reduced.
However, in the portable radio communication device 30, since the length L5 between the short circuit forming end and the electrically opened end of the conductive plate 32 depends on the radio communication frequency in use, the length L5 may be too large, which prevents a liquid crystal display or a keypad for operation from being appropriately arranged on a front surface of the portable radio communication device 30.